The present invention relates to an anaerobic fluidized bed and, more particularly, relates to an anaerobic fluidized bed using micro carriers and improved to make its start-up extremely easy and fast by utilization of an aggregation phenomenon between the carriers.
Anaerobic fluidized bed has been used as one of apparatus for treating municipal and industrial wastewaters. As well known, carriers in the reaction tank come to be fluidized and be a so-called fluidized bed by introducing raw wastewater into the reaction tank upward. Generally, in order to obtain upflow velocity required for fluidization, a part of treated wastewater is recycled from the upper portion of the reaction tank to the lower portion thereof.
In this type of anaerobic fluidized bed, anaerobic bacteria, exist in the seed materials, such as sewage digester sludge, gradually attach onto the surfaces of the carriers and form biofilms through the pass of organic wastewaters.
Relatively large-sized granular materials have been used as such fluidized carriers.
In this specification, a carrier in a state that organisms have not yet attached thereto is named "nucleus" and a carrier in a state that organisms have attached thereto is named "attached granule".
Microorganisms attached onto a carrier are lower in specific gravity than the carrier (nucleus). Accordingly, as the attachment of microorganisms progresses, the apparent density of the attached granule decreases.
Generally, in the fluidized bed, the increase in diameter of fluidized granules functions as a factor causing the increase of the upflow velocity required for fluidization, and the decrease in specific gravity of fluidized granules functions as a factor causing the decrease of the velocity for fluidization.
In the anaerobic fluidized bed, the decrease in specific gravity of granules due to the attachment of microorganisms dominates the function of increasing the velocity required for fluidization by the increase in diameter of granules. Accordingly, with the progress of the attachment of microorganisms, the bed height of the fluidized bed increases to thereby result in washing-out of carriers (attached granules).
To avoid such washing-out of carriers, conventionally, an operation of reducing upflow velocity has been made with the progress of the attachment of microorganisms to nuclei. Specifically, the measure to prevent washing-out of carriers has been made by either of two kinds of operational methods, one being such that the upflow velocity is gradually reduced with the increase of biofilm thickness to thereby keep the bed height nearly constant and the other being such that the upflow velocity is reduced to a value at a steady state when the bed height reaches a predetermined height.
The former operational method, in which the upflow velocity is gradually reduced, however, has a disadvantage that handling is troublesome because the operation of reducing the upflow velocity should be made on the ground of continuously monitoring the bed height. The latter operational method, in which the upflow velocity is reduced to a value at a steady state when the bed height reaches a predetermined height, also has a disadvantage that the cost of assembling is wastefully great because a sufficiently larger board should be provided at the upper portion of the reaction tank.
Alternatively, there has been proposed a further method of removing biofilms from the carriers having the biofilms excessively attached thereto at the inside or outside of the reaction tank by mechanical means to thereby control biofilm thickness. The method of removing excess biofilms from the carriers by mechanical means has a disadvantage that maintenance is difficult because of the complexity of assebling under extremely corrosive conditions.